Polyarylene sulfide (hereinafter sometimes referred to as “PAS”), typified by polyphenylene sulfide (hereinafter sometimes referred to as “PPS”) is an engineering plastic with excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability and other properties, and is widely used as a material for resin parts in a wide range of fields such as electrical/electronic equipment, automobile equipment, and chemical equipment.
A known representative example of a PAS production method includes a method of performing a polymerization reaction on a sulfur source and a dihalo aromatic compound (also abbreviated as “DHA” hereafter) such as p-dichlorobenzene (also abbreviated as “pDCB” hereafter) in an organic amide solvent such as N-methyl-2-pyrrolidone (also abbreviated as “NMP” hereafter).
In recent years, demands for quality and demands from the market for reductions in production cost have spurred research into cost reduction by means of the recovery and reuse of raw materials used in the production of PAS—that is, the recovery and reuse of (i) an organic amide solvent and (ii) an organic solvent used to wash the PAS or to recover the organic amide as desired.
However, the recovery and reuse of unreacted DHA (in particular, pDCB) discharged in the dehydration step during the polymerization reaction and/or remaining in the polymerization reaction solution after the polymerization reaction have not been performed sufficiently.
The present inventors analyzed the reasons thereof and hypothesized that there are the following four causes.
The first cause is related to cost effectiveness. The conversion ratio of DHA in the production of PAS ordinarily exceeds 90%, so the amount of unreacted DHA remaining in the polymerization reaction solution after the polymerization reaction is too small to be recovered and reused. Therefore, if the cost of recovering a small amount of unreacted DHA (for example, the thermal energy cost for distillation or the operational cost) is too high, the cost effectiveness becomes low.
The second cause is the quality or the effect thereof on production when recovered unreacted DHA is reused to produce PAS (also simply abbreviated as “stable PAS production” hereafter). That is, in order to accommodate demands for quality from the market, a wide variety of manufactured goods must be produced. As a result, the starting raw materials, the polymerization reaction conditions, and the like become diverse, and unreacted DHA that is recovered in the recovery/reuse step differs in terms of the types or amounts of impurities that are produced. Therefore, it is natural to assume that the quality of unreacted DHA caused by these impurities or the effects thereof on the PAS production method will differ.
The third cause is related to highly efficient operability in the recovery step. That is, in recovery in the unreacted DHA recovery step during the polymerization reaction for each production formula or in accordance with the types or amounts of impurities after the polymerization reaction, it is considered to be necessary, of course, to control the continuing/stopping of the operation of the recovery device as well as the operation conditions in accordance with the types or amounts of impurities.
Accordingly, even if recovery technology were developed to allow processing, it is difficult to assess whether continuous operability would be stably achieved with the high efficiency required for cost reduction.
The fourth cause is that when taking into consideration the low cost effectiveness described above, it is possible to overcome environmental problems at relatively low cost by disposing unreacted DHA (for example, incineration) as a crude recovered product obtained when recovering a discharge liquid or discharge steam in a dehydration step during the polymerization reaction, a separated liquid in a separation step, a washing discharge liquid in a PAS washing step, an organic solvent used to recover an organic amide solvent, and the like rather than recovering and/or reusing unreacted DHA.
Japanese Unexamined Patent Application Publication No. S61-53325A (Patent Document 1) proposes a method for recovering, distilling, and purifying a solvent from a PAS reaction solution slurry, the method including supplying a PAS reaction solution slurry into a stirring tank of a rectification column having a stirring tank with a vertical jacket as a distillation still, heating the slurry while stirring, and fractionating the evaporating components with the rectification column. It is described that in the initial stage of recovery, water and then a small amount of an unreacted aromatic halide are first fractionated in terms of the order of boiling points, and the rest involves the recovery of polar solvents, which constitutes most of the content. However, Patent Document 1 describes that total reflux is first carried out in the rectification column and is then additional reflux is carried out after the temperature inside the column stabilizes, that the first distillation is received with a receptacle, and that after the completion of water distillation, the pressure inside the column is reduced to obtain a pDCB/NMP solution, so the method is not a method of recovering pDCB with high purity.
Japanese Unexamined Patent Application Publication No. 2008-266181A (Patent Document 2) proposes a PAS production method in which a recovered solvent containing at least water, pDCB, NMP, and NaCl following the recovery of PAS from a PAS polymerization solution is extracted with n-hexanol, and pDCB is then recovered from a water-containing mixture containing at least water, n-hexanol, pDCB, and NMP and used as a raw material for PAS. However, in this method, a step of separating and purifying a mixture containing the four components of NMP, n-hexanol, pDCB, and water obtained as an extracted and recovered product becomes necessary. Therefore, the equipment cost for constructing a distillation device becomes high, whereas the purity of the pDCB that is separated and purified is only 99.6%, which is not satisfactory industrially.
Japanese Unexamined Patent Application Publication No. 2010-144085A (Patent Document 3) proposes a method of recovering unreacted pDCB by means of azeotropy with water from an NMP solution following the separation and recovery of PAS from a polymerization reaction solution. However, in this method, it is absolutely necessary to adjust the water content by adding water to the NMP solution following the separation and recovery of PAS prior to the azeotropy operation. In addition, when unreacted pDCB that is recovered is reused, it cannot be considered sufficiently clear whether the four problems described above will be resolved—in particular, whether PAS can be stably produced.
Japanese Unexamined Patent Application Publication No. H10-007798A (Patent Document 4) proposes a method of returning a distilled polyhalo-aromatic compound to a rectification column as a reflux in order to dramatically reduce the amount of the polyhalo-aromatic compound distilled to the outside of the reaction system in a dehydration step during the polymerization reaction. However, with this method, it is not possible to completely eliminate the discharge of the polyhalo-aromatic compound, and impurities in the polyhalo-aromatic compound returned as a reflux cannot be considered to be sufficiently controlled.
Japanese Unexamined Patent Application Publication No. H10-158399A (Patent Document 5) proposes a method of using a rectification column with an intercooler in order to prevent a polyhalo-aromatic compound from being distilled to the outside of the reaction system in a dehydration step during the polymerization reaction. However, with this method, a rectification column with a complex structure becomes necessary, so the equipment cost becomes high, and the thermal energy cost cannot be considered satisfactory due to the use of the intercooler.
The present inventors searched for a simple method for recovering and reusing unused DHA so as to enable the stable production of PAS.